Melt butter in double boiler or sauce pan. Add flour, seasoning and stir smooth. Slowly add the hot milk stirring constantly. When it thickens melt the grated cheese in the sauce. Into a butteredbaking dish or casserole put layers of the sliced potatoes, onions and cheese sauce, repeating until all ingredients are used. Bake in a moderate oven (350-f) for 1 hour.